You pressed Ctrl+S instead of Ctrl+O to save the file. Ctrl+S is an old flow-control key combination to pause the transmission, and stop scrolling, of data to a terminal (internally, the code sent is called XOFF). Ctrl+Q (XON) is the complement to start transmission and resume scrolling.

Nano ignores these code since it doesn't use scrolling. The muttering is likely a result of its frustration that you didn't remember to use Ctrl+O to save.